Microbial fuel cells offer the prospect of harvesting electricity from organic waste and renewable biomass. These are attractive sources of energy because they are ‘carbon-neutral’; the oxidation of the organic matter only releases recently fixed carbon back into the atmosphere microbial fuel cells could fill a niche that is significantly different from that of the better known abiotic hydrogen- and methanol-driven fuel cells. However, microbial fuel cells do not require fuels that are toxic or explosive or expensive catalysts. Microbial fuel cells can oxidize a diverse range of ‘dirty’ fuels that are often of little perceived value, such as organic waste and the organic matter in soils and sediments. See, e.g., Lovley, D. R., 2006, Bug juice: harvesting electricity with microorganisms, Nature Reviews Microbiology, 4: 497-508.
Electricigens are microbes that conserve energy to support growth by completely oxidizing organic compounds to carbon dioxide with direct electron transfer to the anodes of microbial fuel cells. Electricity production with electricigens is significantly different from that of other types of microorganisms. See, e.g., Lovley, D. R & Nevin, K. P., 2008, Electricity Production with Electricigens, pp. 295-306 in Wall, J., et al., ed. Bioenergy, ASM Press, Washington, D.C. Electricigens have the ability to oxidize organic compounds to carbon dioxide with an electrode serving as the sole electron acceptor, providing high coulombic efficiency that is not available with other microbes. Since electricigens conserve energy for maintenance and growth from electron transfer to anodes, electricigen-powered microbial fuel cells have long-term sustainability. Electricigen-based microbial fuel cells have been run for more than 2 years without a decline in power output.
The most heavily studied electricigens are in the family Geobacteraceae (Lovley, D. R., & K. P. Nevin, 2008. Chapter 23: Electricity production with electricigens. In J. Wall et al. (ed.), Bioenergy. ASM Press, Washington, D.C. pp. 295-306). Desulfuromonas species, found in marine sediments, and Geobacter species, found in freshwater sediments, have similar physiologies, oxidizing short-chain fatty acids to carbon dioxide with Fe(III) oxides serving as the electron acceptor. Geobacteraceae species that have been shown to oxidize acetate with an electrode serving as the sole electron acceptor include Desulfuromonas acetoxidans, Geobacter metallireducens, and Geobacter sulfurreducens. 
Genetic engineering approaches to improve the efficiency of current production in microbial fuel cells by inducing increased rates of respiration in a Geobacter sulfurreducens strain did not result in more current in microbial fuel cells than that produced by wild-type cells (Izallalen, M., et al., 2008, Geobacter sulfurreducens strain engineered for increased rates of respiration, Metab Eng.; 10(5): 267-75). However, it has been found that genetically modified strains of electricigenic microbes that are specifically adapted for improved production of electrical current in microbial fuel cells can be selected and maintained.